1. Field of the Invention
The present invention relates to polarization-independent optical switches/modulators and method for fabricating the same.
2. Description of the Prior Art
Optical switches used in the field of optical communications and optical transmission for switching optical paths are broadly divided into two categories of mechanical switches which switch optical paths by electro-mechanical movements of micro-optic elements and waveguide switches which switch optical paths in the substrates making use of the electrooptic effect or the like. Ones generally have some merits of low insertion loss and good crosstalk characteristics and relative insensitivity to wavelength or polarization. But they have a fatal defect that is not well suited to the construction of large and complex switch array, because they are slow in operation and physically bulky. While the others are potentially broad band, so waveguide switches and modulators are being extensively developed in these days.
Waveguide optical switches and optical modulators using as the substrate an electrooptic crystal having optical anisotropy such as LiNbO.sub.3 have polarization-dependency, and therefore it is required to use only TM polarized light or TE polarized light, or to control both of TM polarized light and TE polarized light by applying a high driving voltage. Accordingly, there is a demand for optical switches and optical modulators being polarization-independent and capable of being driven with a low voltage.
Optical waveguide switches and modulators overcoming the difficulty of polarization-dependency by the use of optical waveguide switches and modulators with the substrate thereof formed of electrooptic crystal are disclosed in U.S. Pat. No. 4,291,939. The polarization-independent optical switch and modulator disclosed in the above patent are characterized by electrode configurations. Sets of electrodes for controlling TE polarized light are disposed on both sides of the optical waveguides and the other sets of electrodes for controlling TM polarized light are mounted on the waveguides. By applying independent voltages to each set of the electrodes, TE polarized light and TM polarized light are individually controlled, and thereby, polarization-independent optical switches and optical modulators are realized.
However, the optical waveguide switch and modulator disclosed in the above described U.S. patent have the electrodes for controlling TE polarized light and electrodes for controlling TM polarized light disposed on the same plane. Therefore, there are points of problem that the length of the portion of parallel optical waveguides is not effectively utilized and the driving circuit becomes complex. In the case of electrode configurations in which, to simplify the driving circuit, one set each of electrodes for controlling TE polarized light and electrodes for controlling TM polarized light are disposed, each set of the electrodes is disposed in different regions of the parallel optical waveguide portion. Therefore, the electrodes for controlling TE polarized light can not be formed especially longer, which results in a problem that a high driving voltage is required.